Preparation process of ultra-low-carbon layered nano-ferrite bainite dual-phase steel

Abstract

The present application relates to the technical field of steel material preparation, and particularly relates to a preparation process of ultra-low-carbon layered nano-ferrite bainite dual-phase steel. The steps comprise: S1, cold rolling of hot-rolled ultra-low-carbon steel sample at room temperature, adjusting the cold rolling parameters to break and lengthen the original bainite and ferrite structure, and obtaining a dense layered structure; S2, rapidly heating the sample to a temperature close to the full austenitization region AC3 temperature and keeping the temperature; S3, rapidly reducing to room temperature to obtain a layered structure composed of a non-recrystallization region and a recrystallization region, the non-recrystallization region being the deformed and lengthened ferrite in the original structure, and the recrystallization region being nano-scale bainite and ferrite. The present application adopts a relatively simple cold rolling + annealing heat treatment process. A layered structure composed of recrystallized nano-ferrite + bainite grains + non-recrystallized ferrite coarse grains is obtained, and the layered structure has excellent properties of high strength and high plasticity.

Description

Technical Field

[0001] This invention relates to the field of steel material preparation technology, and in particular to a preparation process for an ultra-low carbon layered nano-ferrite bainitic dual-phase steel. Background Technology

[0002] With the continuous advancement of science and technology, oil pipelines are evolving towards handling high pressures and increasing pipe diameters, placing increasingly stringent demands on the comprehensive mechanical properties of pipe materials. Specific requirements are set not only for yield strength and uniform elongation, but also for yield-to-tensile strength ratio and hardening index. During the long-distance laying of oil pipelines, they may traverse harsh environments such as permafrost zones, active tectonic zones, debris flow zones, and earthquake-prone areas. To prevent pipeline rupture under large deformations and ensure the safe transportation of oil and natural gas, steel needs to possess high strength, high plasticity, and a low yield-to-tensile strength ratio. Ultra-low carbon steel with a carbon content ≤0.1% exhibits excellent weldability and reduces crack susceptibility. In recent years, with increasingly harsh environments, existing steel materials have become insufficient to meet the performance requirements of pipeline operations.

[0003] Currently, the most effective method for improving the strength of ultra-low carbon steel is grain refinement strengthening. According to the Hall-Page formula, grain size is inversely proportional to mechanical properties. Through large plastic deformation processes, the internal grains of the metal are elongated, broken, and a large number of crystal defects are generated, significantly increasing the internal free energy and drastically reducing the recrystallization temperature, creating favorable conditions for nanocrystals. This results in nanocrystalline materials possessing high strength, but the inverse relationship between strength and plasticity remains unresolved. Gao et al. (Ultrastrong low-carbon nanosteel produced by heterostructure and interstitial mediated warm rolling. Sci. Adv. 6, eaba8169, 2020.) obtained a blocky ultra-strong low-carbon steel with an average thickness of approximately 17.8 nm and a nanosheet-like structure by performing simple industrial hot rolling at 300℃, reducing the thickness by 90%. This resulted in a record-high yield strength of 2.05 GPa and an ultimate strength of 2.15 GPa. Its advantages include a simple process, where the large deformation of warm rolling allows for sufficient microstructure refinement and the formation of a lamellar structure. Its disadvantage is that the elongation of low-carbon steel is not adequately maintained. After large plastic cold deformation, rapid annealing can achieve a comprehensive improvement in mechanical properties. Compared to traditional annealing processes, rapid annealing can delay recrystallization and induce rapid austenite nucleation, significantly refining the grain size during the subsequent austenite transformation. Liu Shichun et al. (Flash annealing enables 1GPa nanoprecipitate-strengthened "NANOHITEN" ferritic steels[J]. Materials Science & Engineering, A. Structural Materials: Properties, Misrostructure and Processing, 2022.) conducted a cold rolling process at 84% followed by rapid short-time annealing at AC1. The high-density, stable nanoprecipitates acted as effective barriers to prevent the growth of recrystallized ferrite and sub-warm austenite on the FA (ferrite core), resulting in a heterogeneous microstructure of ultra-low carbon steel composed of recrystallized ultrafine ferrite grains and non-recrystallized ferrite. The mechanical properties reached a strength level of 1GPa without significant loss of ductility. This method yielded good comprehensive mechanical properties, but the overall performance improvement relied on the differences between phases. However, it was also clearly found that although the above method achieved high yield and tensile strength, the uniform elongation, yield strength ratio, and other indicators could not meet the requirements for large-diameter pipeline steel. Summary of the Invention

[0004] To address the problems existing in the background technology, a preparation process for ultra-low carbon layered nano-ferrite bainitic dual-phase steel is proposed. The preparation steps are as follows:

[0005] S1. The hot-rolled ultra-low carbon steel sample was cold-rolled at room temperature. The cold rolling parameters were adjusted to break and elongate the original bainite and ferrite structure, and a dense layered structure was obtained.

[0006] S2. Rapidly heat the sample to near the AC3 temperature of the fully austenitized region and hold it at that temperature;

[0007] S3. Rapidly cool to room temperature to obtain a layered structure composed of a non-recrystallized region and a recrystallized region. The non-recrystallized region consists of deformed and elongated ferrite from the original microstructure, while the recrystallized region consists of nanoscale bainite and ferrite.

[0008] Preferably, the rolling speed in S1 is 300-350 mm / s, with multiple passes of unidirectional rolling, a reduction of 0.05-0.2 mm per pass, and a total deformation of 70%-90%.

[0009] Preferably, in S2, the temperature is rapidly increased to the fully austenitized AC3 temperature ±20℃ at a heating rate of 50-300℃ / s, and the holding time is 40-120s.

[0010] Preferably, in S3, the temperature is rapidly reduced to room temperature at a rate of 30-200℃ / s.

[0011] Preferably, the hot-rolled ultra-low carbon steel sample used in S1 has the following composition by percentage: C≤0.1%, Si: 0.2-0.6%, Mn: 1.4-1.9%, P≤0.015%, S≤0.025%, Ti+V+Nb≤0.1%, Cr: 0.2-0.4%, Ni: 0.2-0.4%, Mo: 0.2-0.4%, and Cu: 0.2-0.4%, with the balance being iron and other unavoidable impurity elements.

[0012] Compared with the prior art, the present invention has the following beneficial technical effects:

[0013] I. This invention proposes a preparation process for high-strength, high-plasticity, ultra-low-carbon layered nano-ferrite-bainitic dual-phase steel. It introduces a microscopic design concept of heterogeneous structures and employs a relatively simple cold rolling + annealing heat treatment process, resulting in low cost and ease of implementation. After annealing, a lamellar structure is formed, consisting of recrystallized nano-ferrite + bainitic grains + non-recrystallized coarse ferrite grains, representing a uniform layered heterogeneous dual-phase microstructure.

[0014] II. The high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic dual-phase steel prepared by this invention has excellent properties of high strength and high plasticity. When plastic deformation occurs, the deformation of the heterogeneous interface region is uneven, resulting in mutation-induced strengthening (HDI), which in turn increases the yield strength of the material and enhances the strain hardening effect. Its tensile strength reaches 1047 MPa, its yield strength reaches 1019 MPa, its yield ratio is as low as 0.79, its uniform elongation reaches 9%, and its total elongation reaches 18%.

[0015] Third, this invention proposes a method for preparing high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic dual-phase steel that combines high strength and high plasticity. It can be applied to the transportation of oil and natural gas in extremely harsh environments and meets the requirements for high-pressure transportation and large-diameter oil / natural gas pipeline steel. Attached Figure Description

[0016] Figure 1 This is a flowchart of the preparation process of the present invention;

[0017] Figure 2 The mechanical property diagrams involved in Example 1 are shown.

[0018] Figure 3 The mechanical property diagrams involved in Example 2 are shown.

[0019] Figure 4 The mechanical property diagrams involved in Example 3 are shown.

[0020] Figure 5 The mechanical property diagrams involved in Example 4 are shown.

[0021] Figure 6 The mechanical property diagrams involved in Example 5 are shown.

[0022] Figure 7 The mechanical property diagrams involved in Example 6 are shown.

[0023] Figure 8 The mechanical property diagrams involved in Example 7 are shown.

[0024] Figure 9 Transmission electron microscopy image of a high-strength, high-plasticity, ultra-low carbon layered nano-ferrite bainitic dual-phase steel. Detailed Implementation

[0025] Example 1

[0026] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials, with the following chemical composition by weight percentage: C: 0.06%, Si: 0.2%, Mn: 1.6%, P: 0.01%, S: 0.015%, Ti: 0.02%, V: 0.02%, Nb: 0.05%, Cr: 0.26%, Ni: 0.26%, Mo: 0.24%, and Cu: 0.2%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite fully forms when the steel is heated) of this low-carbon steel is 831.19℃.

[0027] Preparation is carried out according to the following Figure 1 The process described involves rolling the sample at room temperature with a mill speed controlled at 300 mm / s, using multiple passes in one direction, with a reduction of 0.1 mm per pass, resulting in a total deformation of 90%. The sample is then rapidly heated to 820°C at a heating rate of 150°C / s and held at that temperature for 50 seconds. After holding, the sample is cooled to room temperature at a cooling rate of 30°C / s to obtain a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0028] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 2 As shown, the prepared duplex steel has a yield strength of 1002.53 MPa, a tensile strength of 1042.40 MPa, a yield-to-tensile ratio of 0.96, a uniform elongation of 7.68%, and a total elongation of 15.65%.

[0029] Example 2

[0030] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials. The chemical composition, by weight percentage, is: C: 0.048%, Si: 0.31%, Mn: 1.48%, P: 0.05%, S: 0.02%, Ti: 0.03%, V: 0.02%, Nb: 0.03%, Cr: 0.31%, Ni: 0.4%, Mo: 0.28%, and Cu: 0.21%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite is fully formed when the steel is heated) of this low-carbon steel is 838℃.

[0031] Preparation is carried out according to the following Figure 1The process described involves rolling the sample at room temperature with a mill speed controlled at 350 mm / s, using multiple passes in one direction. Each pass reduces the sample by 0.05 mm, resulting in a deformation of 70%. The sample is then rapidly heated to 830°C at a rate of 100°C / s and held at that temperature for 50 seconds. After holding, the sample is rapidly cooled to room temperature at a rate of 200°C / s, yielding a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0032] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 3 As shown, the prepared duplex steel has a yield strength of 967.34 MPa, a tensile strength of 1018.31 MPa, a yield-to-tensile ratio of 0.94, a uniform elongation of 9.32%, and a total elongation of 18.73%.

[0033] Example 3

[0034] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials. The chemical composition, by weight percentage, is: C: 0.068%, Si: 0.34%, Mn: 1.81%, P: 0.01%, S: 0.02%, Ti: 0.01%, V: 0.01%, Nb: 0.07%, Cr: 0.26%, Ni: 0.26%, Mo: 0.26%, and Cu: 0.26%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite is fully formed when the steel is heated) of this low-carbon steel is 830℃.

[0035] Preparation is carried out according to the following Figure 1 The process described involves rolling the sample at room temperature with a mill speed controlled at 300 mm / s, using multiple passes in one direction, with a reduction of 0.1 mm per pass, resulting in a total deformation of 80%. The sample is then rapidly heated to 810°C at a heating rate of 200°C / s and held at that temperature for 60 seconds. After holding, the sample is rapidly cooled to room temperature at a cooling rate of 50°C / s to obtain a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0036] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 4 As shown, the prepared duplex steel has a yield strength of 719.25 MPa, a tensile strength of 1016.22 MPa, a yield-to-tensile ratio of 0.71, a uniform elongation of 9.25%, and an elongation of 18.48%.

[0037] Example 4

[0038] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials. The chemical composition, by weight percentage, is: C: 0.068%, Si: 0.34%, Mn: 1.81%, P: 0.01%, S: 0.02%, Ti: 0.01%, V: 0.01%, Nb: 0.07%, Cr: 0.26%, Ni: 0.26%, Mo: 0.26%, and Cu: 0.26%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite is fully formed when the steel is heated) of this low-carbon steel is 830℃.

[0039] Preparation is carried out according to the following Figure 1 The process described involves rolling the sample at room temperature with a mill speed controlled at 300 mm / s, using multiple passes in one direction, with a reduction of 0.2 mm per pass, resulting in a total deformation of 80%. The sample is then rapidly heated to 840°C at a heating rate of 100°C / s and held at that temperature for 50 seconds. After holding, the sample is rapidly cooled to room temperature at a cooling rate of 50°C / s to obtain a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0040] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 5 As shown, the prepared duplex steel has a yield strength of 1019.85 MPa, a tensile strength of 1046.42 MPa, a yield-to-tensile ratio of 0.97, a uniform elongation of 6.76%, and a total elongation of 16.47%.

[0041] Example 5

[0042] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials. The chemical composition, by weight percentage, is: C: 0.068%, Si: 0.34%, Mn: 1.81%, P: 0.01%, S: 0.02%, Ti: 0.01%, V: 0.01%, Nb: 0.07%, Cr: 0.26%, Ni: 0.26%, Mo: 0.26%, and Cu: 0.26%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite is fully formed when the steel is heated) of this low-carbon steel is 830℃.

[0043] Preparation is carried out according to the following Figure 1The process described involves rolling the sample at room temperature with a mill speed controlled at 300 mm / s, using multiple passes in one direction, with a reduction of 0.2 mm per pass, resulting in a total deformation of 80%. The sample is then rapidly heated to 830°C at a heating rate of 100°C / s and held at that temperature for 50 seconds. After holding, the sample is rapidly cooled to room temperature at a cooling rate of 100°C / s to obtain a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0044] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 6 As shown, the prepared duplex steel has a yield strength of 888.927 MPa, a tensile strength of 1047.97 MPa, a yield-to-tensile ratio of 0.84, a uniform elongation of 9.06%, and a total elongation of 18.87%.

[0045] Example 6

[0046] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials. The chemical composition, by weight percentage, is: C: 0.068%, Si: 0.34%, Mn: 1.81%, P: 0.01%, S: 0.02%, Ti: 0.01%, V: 0.01%, Nb: 0.07%, Cr: 0.26%, Ni: 0.26%, Mo: 0.26%, and Cu: 0.26%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite is fully formed when the steel is heated) of this low-carbon steel is 830℃.

[0047] Preparation is carried out according to the following Figure 1 The process described involves rolling the sample at room temperature with a mill speed controlled at 300 mm / s, using multiple passes in one direction, with a reduction of 0.2 mm per pass, resulting in a total deformation of 80%. The sample is then rapidly heated to 830°C at a heating rate of 100°C / s and held at that temperature for 120 seconds. After holding, the sample is rapidly cooled to room temperature at a cooling rate of 100°C / s to obtain a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0048] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 7 As shown, the prepared duplex steel has a yield strength of 776.06 MPa, a tensile strength of 1003.04 MPa, a yield-to-tensile ratio of 0.77, a uniform elongation of 6.32%, and a total elongation of 19.95%.

[0049] Example 7

[0050] This invention proposes a preparation process for ultra-low carbon layered nano-ferritic bainitic dual-phase steel. Hot-rolled ultra-low carbon steel samples are used as raw materials. The chemical composition, by weight percentage, is: C: 0.068%, Si: 0.34%, Mn: 1.81%, P: 0.01%, S: 0.02%, Ti: 0.01%, V: 0.01%, Nb: 0.07%, Cr: 0.26%, Ni: 0.26%, Mo: 0.26%, and Cu: 0.26%, with the balance being iron and other unavoidable impurities. The AC3 line (the temperature at which austenite is fully formed when the steel is heated) of this low-carbon steel is 830℃.

[0051] Preparation is carried out according to the following Figure 1 The process described involves rolling the sample at room temperature with a mill speed controlled at 300 mm / s, using multiple passes in one direction, with a reduction of 0.2 mm per pass, resulting in a total deformation of 80%. The sample is then rapidly heated to 850°C at a heating rate of 100°C / s and held at that temperature for 40 seconds. After holding, the sample is rapidly cooled to room temperature at a cooling rate of 100°C / s to obtain a high-strength, high-plasticity, ultra-low-carbon layered nano-ferritic bainitic duplex steel.

[0052] The duplex steel with a layered nanobainitic-ferrite isomer structure obtained in this embodiment underwent mechanical property testing at room temperature according to national standards. The mechanical properties are as follows: Figure 8 As shown, the prepared duplex steel has a yield strength of 827.32 MPa, a tensile strength of 1039.37 MPa, a yield-to-tensile ratio of 0.79, a uniform elongation of 8.38%, and a total elongation of 16.03%.

[0053] This invention proposes a preparation process for high-strength, high-plasticity, ultra-low-carbon layered nano-ferrite-bainitic dual-phase steel. It introduces a microscopic design concept of heterogeneous structures and employs a relatively simple cold rolling + annealing heat treatment process. During plastic deformation, due to the uneven deformation in the heterogeneous interface region, mutation-induced strengthening (HDI) occurs. Finally, a layered structure consisting of recrystallized nano-ferrite + bainitic grains + non-recrystallized coarse-grained ferrite is obtained. Figure 9 It is a uniform layered heterogeneous two-phase structure with excellent properties of high strength and high plasticity. Its tensile strength reaches 1047MPa, yield strength reaches 1019MPa, yield strength ratio is as low as 0.79, uniform elongation reaches 9%, and total elongation reaches 18%.

[0054] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. Various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention.

Claims

1. A preparation process for ultra-low carbon layered nano-ferrite bainitic dual-phase steel, characterized in that, The preparation steps are as follows: S1. The hot-rolled ultra-low carbon steel sample is cold-rolled at room temperature. The cold rolling parameters are adjusted to break and elongate the original bainite and ferrite structures, resulting in a dense layered structure. The rolling speed in S1 is 300-350 mm / s, with multiple passes of unidirectional rolling. The reduction per pass is 0.05-0.2 mm, and the total deformation is 70%-90%. The hot-rolled ultra-low carbon steel sample used in S1 has the following composition by percentage: C≤0.1%, Si: 0.2-0.6%, Mn: 1.4-1.9%, P≤0.015%, S≤0.025%, Ti+V+Nb≤0.1%, Cr: 0.2-0.4%, Ni: 0.2-0.4%, Mo: 0.2-0.4%, and Cu: 0.2-0.4%, with the balance being iron and other unavoidable impurity elements; S2. Rapidly heat the sample to near the AC3 temperature of the fully austenitized region and hold it at that temperature; in S2, rapidly heat the sample to the AC3 temperature of full austenitization at a heating rate of 50-300℃ / s ±20℃, and hold it for 40-120s. S3 is rapidly cooled to room temperature to obtain a layered structure composed of a non-recrystallized region and a recrystallized region. The non-recrystallized region is the deformed and elongated ferrite in the original microstructure, and the recrystallized region is nanoscale bainite and ferrite. In S3, the temperature is rapidly cooled to room temperature at a rate of 30-200℃ / s.

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